That might work. It is somewhat similar to the Warlock special 'Fiendish Resilience'.
Personally, I think the nonlethal damage is an interesting alternative.
Had I come up with the idea, I likely would have used something akin to:
Benefit: The cleric can at will as a standard action heal a number of hit points equal to up to the level of the prepared spell, but in exchange the cleric takes nonlethal damage equal to the points healed. This non-lethal damage must heal naturally. It cannot be healed magically except by a Miracle or other form of direct intercession from their patron deity.
The fact that this nonlethal damage cannot be healed magically acts as a limit to what otherwise could be an overly potent at will ability.
Actually, if I used the above I would likely go a step further: the ability to heal ability damage on a one for one basis (ie: the cleric takes on the damage, but it must heal naturally). Perhaps two points of ability damage to the cleric to heal one point of ability drain might also work? Or perhaps that is a bit too powerful? Hmm, or perhaps the feat would require the preparing or ability to cast one of the 'Restoration' spells, somehow limiting its ability to heal based on the type of restoration cast?
I much rather prefer limitations that are loose enough that the player doesn't necessarily feel them as such (The cleric could choose to heal hp equal to their total hp and fall unconscious if the player so willed it.) while at the same time are strict enough to prevent overuse of potentially overly potent abilities (The player will almost never choose to do so, however, as this would a) remove the cleric from play for the day, and b) could leave the cleric vulnerable to potentially lethal dangers, dependent on the situation.).